Integration of Catalytic CO2 Oxidation and Oxyfuel Sour Compression

ABSTRACT

Sulfur dioxide (SO 2 ) may be removed from carbon dioxide feed gas by contacting the carbon dioxide at an elevated temperature and an elevated pressure with a catalyst for oxidizing SO 2 , in the presence of oxygen (O 2 ) to convert SO 2  to sulfur trioxide (SO 3 ); contacting SO 3  in the resultant SO 3 -enriched carbon dioxide gas with water to produce sulfuric acid and SO 2 -depleted carbon dioxide gas; and separating the sulfuric acid from the SO 2 -depleted carbon dioxide gas. If present, NO x  is also removed from the carbon dioxide feed gas as nitric acid to produce SO 2 -depleted, NO x -lean carbon dioxide gas. The method has particular application in the removal of SO 2  and NO x  from flue gas produced by oxyfuel combustion of a hydrocarbon fuel or carbonaceous fuel, within or downstream of the CO 2  compression train of a CO 2  recovery and purification system.

BACKGROUND OF THE INVENTION

The present invention relates generally to a method for purifying carbondioxide gas. In particular, the present invention relates to a methodfor removing sulfur dioxide (SO₂) from carbon dioxide gas comprising SO₂as a contaminant. The method also removes NO_(x), if present as afurther contaminant, from the carbon dioxide gas. The invention hasparticular application in the purification of crude carbon dioxide, e.g.flue gas from an oxyfuel combustion process in a pulverized coal firedpower station in which sulfur containing carbonaceous or hydrocarbonfuel is combusted in a boiler to produce steam for electric powergeneration.

The term “SO_(x)” means oxides of sulfur and includes SO₂ and sulfurtrioxide (SO₃). The term “NO_(x),” means oxides of nitrogen and includesprimarily nitric oxide (NO) and nitrogen dioxide (NO₂). NO_(x) maycomprise one or more other oxides of nitrogen including N₂O, N₂O₄ andN₂O₃.

It has been asserted that one of the main causes of global warming isthe rise in greenhouse gas contamination in the atmosphere due toanthropological effects. The main greenhouse gas which is being emitted,carbon dioxide (CO₂), has risen in concentration in the atmosphere from270 ppm before the industrial revolution to the current figure of about378 ppm. Further rises in CO₂ concentration are inevitable until CO₂emissions are curbed. The main sources of CO₂ emission are fossil fuelfired electric power stations and from petroleum fuelled vehicles.

The use of fossil fuels is necessary in order to continue to produce thequantities of electric power that nations require to sustain theireconomies and lifestyles. There is, therefore, a need to deviseefficient means by which CO₂ may be captured from power stations burningfossil fuel so that it can be stored rather than being vented into theatmosphere. Storage may be deep undersea; in a geological formation suchas a saline aquifer; or a depleted oil or natural gas formation.Alternatively, the CO₂ could be used for enhanced oil recovery (EOR).

The oxyfuel combustion process seeks to mitigate the harmful effects ofCO₂ emissions by producing a net combustion product gas consisting ofCO₂ and water vapour by combusting a carbonaceous or hydrocarbon fuel inpure oxygen. This process would result in an absence of nitrogen (N₂) inthe flue gas, together with a very high combustion temperature whichwould not be practical in a furnace or boiler. In order to moderate thecombustion temperature, part of the total flue gas stream is typicallyrecycled, usually after cooling, back to the burner.

An oxyfuel process for CO₂ capture from a pulverised coal-fired powerboiler is described in a paper entitled “Oxy-combustion processes for CO₂ capture from advanced supercritical PF and NGCC power plants” (Dillonet al; presented at GHGT-7, Vancouver, September 2004), the disclosureof which is incorporated herein by reference.

Oxyfuel combustion produces raw flue gas containing primarily CO₂,together with contaminants such as water vapour; “non-condensable”gases, i.e. gases from chemical processes which are not easily condensedby cooling, such as excess combustion oxygen (O₂), and/or O₂, N₂ andargon (Ar) derived from any air leakage into the system; and acid gasessuch as SO₃, SO₂, hydrogen chloride (HCl), NO_(x) and NO₂ produced asoxidation products from components in the fuel or by combination of N₂and O₂ at high temperature. The precise concentrations of the gaseousimpurities present in the flue gas depend on factors such as on the fuelcomposition; the level of N₂ in the combustor; the combustiontemperature; and the design of the burner and furnace.

In general, the final, purified, CO₂ product should ideally be producedas a high pressure fluid stream for delivery into a pipeline fortransportation to storage or to site of use, e.g. in EOR. The CO₂ mustbe dry to avoid corrosion of, for example, a carbon steel pipeline. TheCO₂ impurity levels must not jeopardise the integrity of the geologicalstorage site, particularly if the CO₂ is to be used for EOR, and thetransportation and storage must not infringe international and nationaltreaties and regulations governing the transport and disposal of gasstreams.

It is, therefore, necessary to purify the raw flue gas from the boileror furnace to remove water vapour; SO_(x); NO_(x); soluble gaseousimpurities such as HCl; and “non-condensable” gases such as O₂, N₂ andAr, in order to produce a final CO₂ product which will be suitable forstorage or use.

In general, the prior art in the area of CO₂ capture using the oxyfuelprocess has up to now concentrated on removal of SO_(x) and NO_(x)upstream of the CO₂ compression train in a CO₂ recovery and purificationsystem, using current state of the art technology. SO_(x) and NO_(x)removal is based on flue gas desulfurization (FGD) schemes such asscrubbing with limestone slurry followed by air oxidation producinggypsum, and NO_(x) reduction using a variety of techniques such as lowNO_(x) burners, over firing or using reducing agents such as ammonia orurea at elevated temperature with or without catalysts. ConventionalSO_(x)/NO_(x) removal using desulfurization and NO_(x) reductiontechnologies is disclosed in “Oxyfuel Combustion For Coal-Fired PowerGeneration With CO ₂ Capture—Opportunities And Challenges” (Jordal etal; GHGT-7, Vancouver, 2004). Such process could be applied toconventional coal boilers.

U.S. Pat. No. 4,781,902 discloses a process in which SO₂ and NO_(x) maybe removed from flue gas from combustion processes by a SelectiveCatalytic Reduction (“SCR”) of NO_(x) to N₂ (“deNO_(x)”) using ammonia,followed by an oxidation of SO₂ to SO₃ using vanadium pentoxide-basedcatalysts. Water vapour reacts with SO₃ and is then condensed within aWet gas Sulfuric Acid (WSA) condenser to produce an aqueous solution ofsulphuric acid. The deNO_(x) and SO₂-oxidation reactions take place atelevated temperature, typically about 400° C., and at about atmosphericpressure. 90-95% SO₂ conversion using this process has been calculatedto require a volumetric hourly space velocity in the SO₂ oxidationreactor of about 2500 Nm³ _(feed)/h/m³ _(catalyst bed). The process hasbeen developed by Haldor Topsøe A/S of Lyngby, Denmark and is known asthe SNOX™ process.

US 2004/0071621 A1 discloses a process for the removal of SO₂ from fluegas generated in a combustion process. The flue gas is cooled and sootis removed from the cooled flue gas. The cooled soot-free flue gas isfurther cooled and SO₂ in the further cooled, soot-free, flue gas isoxidized to SO₃ in the presence of O₂ using a catalyst unit containingstructured arrangements of at least one activated carbon fiber board,and is washed with water to produce a dilute (2˜5%) aqueous solution ofsulfuric acid. The washed gas is then discharged directly to theatmosphere via a mist eliminator (which is optional) and a smoke stack.It is disclosed that the pressure of the flue gas is elevated to causethe gas to pass through the catalyst unit.

US 2007/0122328 A1 (granted as U.S. Pat. No. 7,416,716 B1) discloses thefirst known method of removing SO₂ and NO_(x) from crude carbon dioxidegas produced by oxyfuel combustion of a hydrocarbon or carbonaceousfuel, in which the removal steps take place in the CO₂ compression trainof a CO₂ recovery and purification system. This process is known as a“sour compression” process since acid gases are compressed with carbondioxide flue gas. The method comprises maintaining the crude carbondioxide gas at elevated pressure(s) in the presence of O₂ and water and,when SO₂ is to be removed, NO_(x), for a sufficient time to convert SO₂to sulfuric acid and/or NO_(x) to nitric acid; and separating saidsulfuric acid and/or nitric acid from the crude carbon dioxide gas.

There is a continuing need to develop new methods for removing SO₂ and,where present, NO_(x) from carbon dioxide gas, and particularly fromcrude carbon dioxide gas such as flue gas produced in an oxyfuelcombustion process such as that involved in a pulverized coal-firedpower boiler.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to develop a new method forremoving SO₂ and, where present, NO_(x) from carbon dioxide gas,particularly from flue gas from an oxyfuel combustion process.

It is an object of preferred embodiments of the present invention toreduce the size of, or even eliminate, (i) conventional FGD systems toremove SO₂, and/or (ii) conventional SCR systems to remove NO_(x).

It is another object of preferred embodiments of the present inventionto improve the methods disclosed in US 2007/0122328 A1 by (i) enabling areduction in the size of the sour compression reactor system, and/or(ii) accelerating SO₂ (and NO) oxidation to SO₃ (and NO₂), and/or (iii)reducing the amount of (mixed) acid condensate that is produced and,therefore, would have to be processed and/or disposed of.

It is a further object of preferred embodiments of the present inventionto reduce the catalyst requirement in conventional processes, e.g. theSNOX™ process by Haldor Topsøe A/S, for the removal of SO₂ from flue gasby oxidation to SO₃ and condensation with water to form aqueous sulfuricacid.

According to the first aspect of the present invention, there isprovided a method for removing SO₂ from a carbon dioxide feed gascomprising SO₂ as a contaminant, said method comprising:

-   -   contacting said carbon dioxide feed gas at an elevated        temperature and an elevated pressure with a catalyst for        oxidizing SO₂, in the presence of O₂ to convert SO₂ to SO₃ and        produce an SO₃-enriched carbon dioxide gas;    -   contacting SO₃ in said SO₃-enriched carbon dioxide gas with        water to produce sulfuric acid and a SO₂-depleted carbon dioxide        gas; and    -   separating said sulfuric acid from said SO₂-depleted carbon        dioxide gas, or from an SO₂-depleted carbon dioxide gas derived        therefrom.

The present method has particular application in removing SO₂ and NO_(x)from flue gas generated by oxyfuel combustion of hydrocarbon fuel orcarbonaceous fuel.

The present method substantially reduces the concentration of SO₂ and,where present, NO_(x) in carbon dioxide gas such as flue gas. The methodcan be integrated with a conventional FGD and/or SCR system therebysignificantly reducing the size of these systems. Indeed, the method canbe used to replace such systems. The method can also be readilyintegrated with the method disclosed in US 2007/0122328 A1 therebyreducing of the size of the sour compression reactor system. Suchembodiments have the added benefit of reducing the amount ofSO₂-oxidation catalyst required as compared to the SNOX™ process, due tothe dual mechanisms by which SO₂ is oxidized.

According to a second aspect of the present invention, there is providedapparatus for removing SO₂ from carbon dioxide feed gas comprising SO₂as a contaminant, said apparatus comprising:

-   -   a compressor arrangement for elevating the pressure of said        carbon dioxide feed gas comprising SO₂ as a contaminant;    -   a catalytic reactor system comprising a catalyst for oxidizing        SO₂, said reactor system being suitable for contacting said        carbon dioxide feed gas at an elevated temperature and an        elevated pressure with said catalyst in the presence of O₂ to        convert SO₂ to SO₃ and produce SO₃-enriched carbon dioxide gas;    -   a conduit arrangement for feeding carbon dioxide gas at said        elevated pressure from the compressor arrangement to the        catalytic reactor system;    -   a separator system for contacting SO₃ in said SO₃-enriched        carbon dioxide gas with water to produce sulfuric acid and        SO₂-depleted carbon dioxide gas, and for separating said        sulfuric acid from said SO₂-depleted carbon dioxide gas, or from        a SO₂-depleted carbon dioxide gas derived therefrom; and    -   a conduit arrangement for feeding SO₃-enriched carbon dioxide        gas from the catalytic reactor system to the separator system.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a flow sheet depicting one embodiment of the presentinvention;

FIG. 2 is a flow sheet depicting a second embodiment of the presentinvention; and

FIG. 3 is a flow sheet depicted a third embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The method for removing SO₂ from a carbon dioxide feed gas comprisingSO₂ as a contaminant comprises contacting the carbon dioxide feed gas atan elevated temperature and an elevated pressure with a catalyst foroxidizing SO₂, in the presence of O₂ to convert SO₂ to SO₃ and producean SO₃-enriched carbon dioxide gas. SO₃ in the SO₃-enriched carbondioxide gas is contacted with water to produce sulfuric acid and aSO₂-depleted carbon dioxide gas. Sulfuric acid is separated fromSO₂-depleted carbon dioxide gas, or from an SO₂-depleted carbon dioxidegas derived therefrom.

The method is primarily intended as an alternative or improved method tothat disclosed in US 2007/0122328 A1 for removing SO₂ and NO_(x) fromflue gas generated by oxyfuel combustion of a hydrocarbon orcarbonaceous fuel, in or downstream of, a CO₂ compression train in a CO₂recovery and purification system.

It should be noted that the percentages indicated for the variouscomponents in gas streams discussed below are approximate molarpercentages (mol. %) calculated on a dry basis. In addition, allpressures provided below are absolute pressures and not gauge pressures.

The method is understood by the Inventors to involve the followingreactions:

2SO₂+O₂→2SO₃  (i)

SO₃+H₂O→H₂SO₄  (ii)

The method typically removes over 80% of the SO₂ contaminant in thecarbon dioxide feed gas and, in most embodiments, the method removesover 90% of the SO₂ contaminant in the feed gas. In some embodiments,the method removes substantially all (e.g. >95%) of the SO₂ contaminantin the carbon dioxide feed gas to produce a substantially SO_(x)-freecarbon dioxide gas.

The method is suitable to purify carbon dioxide containing SO₂ as acontaminant from any source. However, in preferred embodiments, thecarbon dioxide gas is, or is derived from, flue gas produced bycombustion of a fuel selected from the group consisting of hydrocarbonfuels such as natural gas, and carbonaceous fuels such as coal. Themethod has particular application for removing SO₂ from flue gasproduced by oxyfuel combustion of a sulfur-containing fuel, particularlycoal.

Flue gas generated in an oxyfuel combustion process usually containscarbon dioxide as the major component, with SO_(x), NO_(x) and thenon-condensable gases O₂, N₂, Ar, Kr and Xe. SO_(x) is produced by thecombustion of elemental sulfur and/or sulfur-containing compoundspresent in the fuel. O₂ is present in the flue gas from excess O₂ usedin the combustion and from air ingress into the combustion unit which isalso responsible for the presence of N₂, Ar, Kr and Xe in the flue gas.NO_(x) is produced by reaction N₂ with O₂ in the combustion unit.

Further components in the flue gas include solid particulates such asfly ash and soot; water; CO; HCl; CS₂; H₂S; HCN; HF; volatile organiccompounds (VOCs) such as CHCl₃; metals including mercury, arsenic, iron,nickel, tin, lead, cadmium, vanadium, molybdenum and selenium; andcompounds of these metals.

Flue gas from the combustor is typically washed with water to removeparticulates (such as soot and/or fly ash) and water soluble components(such as HF, HCl and/or SO₃). Additionally, the flue gas may befiltered, using equipment such as a baghouse or electrostaticprecipitator, to enhance particulate removal. Since the flue gas istypically at atmospheric pressure, it is then compressed after washingto the elevated pressure to form the carbon dioxide feed gas to bepurified by the method. However, if the feed gas originates from asource, such as a pressurized oxyfuel combustion system, that is alreadyat the required elevated pressure, then compression is not required.

Where the carbon dioxide gas is produced in an oxyfuel combustionprocess, the method usually involves the combustion of the fuel in pureO₂ or an O₂-rich gas, e.g. a gas comprising at least 80% O₂, optionallywith recycled flue gas from the combustion process to moderate thetemperature of combustion and control heat flux.

The method may be used to remove SO₂ and, optionally, NO_(x) from carbondioxide feed gas having a flow rate from 200 kmol/h to 40,000 kmol/hwhich flow rates are typical for flue gas generated in an oxyfuelcombustion process.

The method may be used to remove SO₂ from a stream of otherwise pure CO₂gas. However, the method has particular application in removing SO₂ from“impure” carbon dioxide gas, e.g. carbon dioxide gas having from about90% to about 95% CO₂, and more particularly in removing SO₂ from “crude”carbon dioxide gas, e.g. carbon dioxide feed gas having from about 40%to about 90% CO₂, such as flue gas from an oxyfuel combustion process.In preferred embodiments, the carbon dioxide feed gas has from about 60%to about 90% CO₂; and preferably from about 65% to about 85% CO₂.

The amount of SO₂ contaminant in the feed gas is usually more than 50ppm. The amount of SO₂ contaminant in the feed gas is usually no morethan about 10,000 ppm. The amount of SO₂ contaminant in the feed gas istypically from about 100 ppm to about 5,000 ppm.

O₂ may be added to the feed gas to provide the O₂ necessary to oxidizeSO₂ to SO₃. However, in embodiments where the carbon dioxide feed gasis, or is derived from, flue gas from a combustion process, at leastsufficient (and often excess) O₂ is usually present in the carbondioxide feed gas such that additional O₂ from an external source is nottypically required. In such embodiments, the amount of O₂ in the feedgas is usually from about 0.1% to about 15%, e.g. from about 1% to about8%, of the feed gas.

The term “elevated pressure” is intended to mean a pressure that issignificantly greater than atmospheric pressure. For example, the termis intended to exclude minor elevations in pressure over atmosphericpressure, such as those elevations provided by a blower or fan in orderto force a gas through apparatus operating at about atmosphericpressure. Such minor pressure elevations are considered to beinsignificant in the context of the present invention.

The elevated pressure is usually at least 2 bar (0.2 MPa), e.g. at least3 bar (0.3 MPa), or at least 5 bar (0.5 MPa). The elevated pressure isusually no more than about 100 bar (10 MPa) and preferably no more thanabout 50 bar (5 MPa). The elevated pressure may be from about 3 bar toabout 50 bar (0.3 MPa to 5 MPa), e.g. from about 5 bar to about 50 bar(0.5 MPa to 5 MPa), or from about 10 bar to about 40 bar (1 MPa to 4MPa).

The term “elevated temperature” is intended to mean a temperature thatis significantly greater than ambient temperature. The American Societyof Testing and Materials (ASTM) defines ambient temperature as from 50°F. to 100° F., i.e. from about 10° C. to about 38° C.

The elevated temperature is typically at least 300° C. The elevatedtemperature is usually no more than about 700° C. The elevatedtemperature may be from 300° C. to about 700° C., e.g. from 300° C. toabout 600° C. In some embodiments, the elevated temperature is fromabout 375° C. to about 475° C.

The SO₃ in the SO₃-enriched carbon dioxide gas is contacted with waterat an elevated pressure. The elevated pressure of this contact step isusually the same as the elevated pressure of the catalytic oxidationstep, subject to any inherent pressure drop within the method orapparatus. However, it is conceivable that there may be a desire tocarry out the SO₃/water contact step at a “second” elevated pressurethat is different from a “first” elevated pressure of the catalyticoxidation step. The second elevated pressure may be higher or lower thanthe first elevated pressure, but would within the preferred ranges forthe elevated pressure. Suitable pressure adjustment arrangements may beused to increase or lower the elevated pressure as required, as is knownin the art.

At preferred elevated temperatures, SO₃ will react with water vapor toproduce sulfuric acid. Without wishing to be bound by any particulartheory, the Inventors believe that, provided the elevated temperature issufficiently high, this reaction takes place in the gas phase to produceeither gaseous sulfuric acid or sulfuric acid in the form of an acidmist or aerosol, depending on the temperature. In some embodiments,sulfuric acid may be separated from SO₂-depleted carbon dioxide gaswithout first cooling the gas mixture to the point where the acidcondenses. However, since the sulfuric acid in these embodiments iseither gaseous or an aerosol, these embodiments may not be preferred.

In preferred embodiments, the method comprises cooling the SO₃-enrichedcarbon dioxide gas to a reduced temperature that is less than theelevated temperature and no more than the acid dew point at the elevatedpressure, thereby condensing sulfuric acid as a liquid, usually in theform of an aqueous acid solution. The sulfuric acid may then beseparated from the SO₂-depleted carbon dioxide gas, or from aSO₂-depleted carbon dioxide gas derived therefrom, using conventionalmethods for separating gas and liquid phases.

The “acid dew point” is a conventional term in the art referring to thetemperature at which reaction conditions favor production of inorganicacid as a liquid, for example from the gas phase equilibrium reaction ofSO₃ and water. The acid dew point is dependent on pressure and theconcentration of other components such as SO₃ (and NO_(x)), and a higherpressure (or a higher concentration of the other component(s)) means ahigher dew point. Table 1 provides some examples from the literature(Oil & Gas Journal; Vol. 108; Issue 7; 22 Feb. 2010) of how acid dewpoint varies with pressure, water and SO₃ concentrations.

TABLE 1 Pressure Dew point (° C.) Dew point (° C.) Dew point (° C.) bar5% H₂O; 5,000 20% H₂O; 5,000 5% H₂O; 10,000 (MPa) ppm SO₃ ppm SO₃ ppmSO₃   1 (0.1) 194 204 201 10 (1) 233 242 240 30 (3) 250 259 257

The reduced temperature is typically no more than 300° C. and is usuallyfrom ambient temperature to about 275° C. The reaction temperature maybe more than ambient temperature, e.g. at least 40° C., and may be fromabout 40° C. to about 275° C. Preferred ranges for the reactiontemperature may be from ambient temperature to 150° C., or from about20° C. to about 100° C.

The SO₃-enriched carbon dioxide gas may be cooled by indirect heatexchange against at least one coolant. The coolant may be a liquid, e.g.water, or gaseous, e.g. air. In preferred embodiments, the gas may becooled initially using a first coolant, and then further cooled using asecond coolant. The first and second coolants may be the same ordifferent. In a preferred embodiment, the gas is cooled initially byindirect heat exchange against water, and then further cooled byindirect heat exchange using air. An example of a suitable air-cooledheat exchanger is a shell-and-tube type acid condenser such as the WSAcondenser used in the SNOX™ process.

All of the water vapor required to react with the SO₃ produced by thecatalytic oxidation of SO₂, may be provided internally, e.g. having beenproduced in a combustion process and already being present as a furthercontaminant of the carbon dioxide feed gas, and/or added in a flue gaswashing step. However, water from an external source may be added to theSO₃-enriched carbon dioxide gas, particularly at the start up of themethod. Water may be added in vapor form but, in preferred embodiments,water is added as a liquid. If water is added from an external source,it usually helps to cool the gas, particularly where water is added as aliquid. Thus, the SO₃-enriched carbon dioxide gas may be cooled bydirect heat exchange with water from an external source. Water added inthis way helps ensure that no acid is carried downstream to corrodeapparatus.

Where sulfuric acid is produced as an aqueous acid solution, theconcentration of sulfuric acid in the aqueous acid solution will dependon the amount of water present in the carbon dioxide feed gas and/or theamount of water added from an external source. However, the aqueous acidsolution typically comprises from 50 wt % to 99.9 wt sulfuric acid.Preferably, the aqueous acid solution comprises no less than 75 wt %and, more preferably, no less than 90 wt % sulfuric acid. In preferredembodiments, the aqueous acid solution is concentrated sulfuric acid,i.e. >95 wt % sulfuric acid.

In some embodiments, the carbon dioxide feed gas comprising SO₂ does notalso comprise NO_(x). An example of such an embodiment is where thecarbon dioxide gas is flue gas from a combustion process where NO_(x)has already been removed, for example by a deNOx step, after suitablepressure and/or temperature adjustment. However, in preferredembodiments, the carbon dioxide feed gas comprising SO₂ as acontaminant, comprises NO_(x) as a further contaminant. In theseembodiments, SO₂ and NO are converted to SO₃ and NO₂ respectively, andthe method additionally produces nitric acid for separation withsulfuric acid from the SO₂-depleted carbon dioxide gas which is alsoNO_(x)-lean.

NO is converted to nitric acid in the presence of O₂ and water to nitricacid by the following series of reactions:

2NO+O₂

2NO₂  (iii)

2NO₂+H₂O

HNO₂+HNO₃  (iv)

3HNO₂

HNO₃+2NO+H₂O  (v)

NO₂ also oxidizes SO₂ non-catalytically to form SO₃ according to thefollowing formula:

NO₂+SO₂

NO+SO₃  (vi)

Reactions (ii) to (vi) are referred to herein as the “sour compression”reactions. Following extensive studies (Counce, R. M. (1977), “Aliterature review of nitrogen oxide absorption into water and dilutenitric acid”, Technical Report ORNL/TM-5921, Oak Ridge NationalLaboratory), it has been determined that the rate of reaction (i) isincreased as the reaction pressure increases. The Inventors realizedthat carrying out the present method at elevated pressure improves therate of reaction (i). In particular, the elevated pressure in theseembodiments is preferably at least about 3 bar, which the Inventors havedetermined is the pressure threshold at which the rate of reaction (i)becomes commercially more useful.

Further details of the sour compression reactions and of suitable sourcompression reactor systems are provided in US 2007/0122328 A1, thedisclosure of which is incorporated herein by reference.

In preferred embodiments of the present invention, the method comprisesmaintaining the SO₃-enriched carbon dioxide gas comprising NO_(x) atelevated pressure(s), and preferably at said reduced temperature, in thepresence of O₂ and water for a period of time sufficient to convertNO_(x) to nitric acid. The elevated pressure(s) for the sour compressionreactions is preferably at least about 3 bar (0.3 MPa), e.g. at leastabout 5 bar (0.5 MPa). The elevated pressure(s) is usually no more than100 bar (10 MPa), and preferably no more than 50 bar (5 MPa). Inpreferred embodiments, the elevated pressure(s) is from about 5 bar toabout 50 bar (0.5 MPa to 5 MPa).

The Inventors have realised that the presence of the NO_(x) in thecarbon dioxide gas comprising SO₂, assists in the conversion of SO₂ toSO₃, thereby enabling the simultaneous production of sulfuric acid bytwo different mechanisms, i.e. (a) by the heterogeneous catalyticreaction (i) followed by reaction (ii), and (b) by the non-heterogeneouscatalytic reaction (vi) followed by reaction (ii). In this way, not onlyis the overall rate of conversion of SO₂ to SO₃ greater in theseembodiments of the present invention than in conventional processeswhich rely solely on heterogeneous catalytic reaction (i) to oxidise SO₂to SO₃ (e.g. the SNOX™ process), but also the heterogeneous oxidationcatalyst requirement in the present invention is greatly reduced to thatin those conventional processes.

In preferred embodiments of the invention, the feed gas is contactedwith the catalyst in a catalytic reactor at a volumetric hourly spacevelocity from about 5,000 to about 500,000 Nm³ _(feed)/h/m³_(catalyst bed), e.g. from 10,000 to 200,000 Nm³ _(feed)/h/m³_(catalyst bed).

At preferred elevated temperatures, NO_(x) will be converted in the gasphase in the presence of O₂ and water in accordance with reaction (iii)to (v) to produce nitric acid in gaseous or aerosol form depending onthe temperature. Therefore, in some embodiments, nitric acid may beseparated, together with sulfuric acid, from SO₂-depleted, NO_(x)-leancarbon dioxide gas without first cooling the gas mixture to or below theacid dew point under the particular conditions at hand. However, sincethe acids in these embodiments are either gaseous or in aerosol form,these embodiments may not be preferred.

In preferred embodiments, the method comprises cooling the SO₃-enrichedcarbon dioxide gas comprising NO_(x), to a reduced temperature that isless than the elevated temperature and no more than the acid dew pointat the elevated pressure, thereby condensing a mixture of nitric andsulfuric acids, usually in the form of an aqueous mixed acid solution.Typically, the SO₃-enriched carbon dioxide gas comprising NO_(x) iscooled to a reduced temperature of no more than 300° C. and, usually, toa reduced temperature from ambient temperature to about 275° C. Themixture of nitric and sulfuric acids may then be separated from theSO₂-depleted, NO_(x)-lean carbon dioxide gas, or from a SO₂-depleted,NO_(x)-lean carbon dioxide gas derived therefrom, in the form of thecondensed liquid using conventional methods for separating gas andliquid phases.

The mixed acid condensate may be removed from the system, pumped andcooled by indirect heat exchange against a coolant before being recycledto the system to help cool the SO₃-enriched carbon dioxide gas andproduce more acid condensate.

Where the feed gas is flue gas from an oxyfuel combustion process andthe acids are produced in the form of an aqueous mixed acid solution,the nitric acid is typically more dilute that the sulfuric acid sincethe SO₂:NO_(x) ratio in the gas is >1. The concentration of nitric acidmay be from about 10 wt % to about 100 wt %.

Residence time in a reactor system (i.e. contact time or “hold up” time)determines the degree or extent of the sour compression reactions. Inthis connection, the period of time required for converting NO_(x) tonitric acid is typically longer than that required for converting SO₂ tosulfuric acid. This period of time is usually more than 5 s, e.g. morethan about 10 s or more than about 20 s. The period of time is usuallyno more than 1000 s, and preferably no more than 600 s. In view of thepresence of the SO₂ oxidation catalyst, the period of time required istypically significantly less than 600 s, e.g. no more than about 200 s,and preferably no more than about 100 s. For example, the period of timemay be from 5 to about 600 s, e.g. from about 10 to about 200 s.

Where the carbon dioxide gas comprising SO₂ as a contaminant, alsocomprises NO_(x) as a further contaminant, the method typically removesat least 40%, e.g. at least about 60% and, in some embodiments, at leastabout 90%, of the NO_(x) contaminant. Preferably, the method removes atleast the bulk of the NO_(x) contaminant, e.g. from 40% to about 99.9%,and, in some preferred embodiments, from about 60% to about 95%.

Where the method is integrated with an oxyfuel combustion process usingcoal as fuel, mercury will typically be present in the carbon dioxidegas as a further contaminant (based on typical coal compositions). Afurther advantage of these embodiments of the present invention is thatany elemental mercury or mercury compounds present as furthercontaminant(s) in the carbon dioxide gas will also be removed, sinceelemental mercury in the vapor phase will be converted to mercuricnitrate and mercury compounds react readily with nitric acid. Typicalnitric acid concentrations in these embodiments of the process will besufficient to remove all of the mercury from the carbon dioxide gas,either by reaction or dissolution.

In some embodiments, the SO₂ oxidation catalyst has no effect on theoxidation of NO to NO₂. However, in preferred embodiments, the SO₂oxidation catalyst also oxidizes NO to NO₂, thereby further promotingthe non-heterogeneous catalytic reactions (iii) to (vi).

The SO₂-oxidation catalyst is preferably selected from the groupconsisting of activated carbon; and oxides of transition metalsincluding vanadium, copper, chromium, manganese, iron and platinum.Vanadium pentoxide is particularly preferred.

In embodiments where the catalyst is activated carbon, the catalyst ispreferably in the form of corrugated sheets of activated carbon fiberboard. In embodiments where the catalyst is a transition metal oxide,the catalyst is typically supported on an inert support, e.g. silica. Insome embodiments, the supported catalyst comprises an alkali metalpromoter such as potassium or cesium. The supported catalyst istypically either in a “loose” form, such as pellets, or plain or shapedrings, or in the form of a structured catalyst, such as a ceramicmonolith.

Particularly preferred catalysts are the transition metal oxidecatalysts disclosed in U.S. Pat. No. 4,781,902, the disclosure of whichis incorporated herein by reference. These catalysts are the VK seriesof catalysts developed by Haldor Topsøe A/S. Details of the VK catalystsare summarized in Table 2.

TABLE 2 VK38 VK48 VK58 VK69 VK-WSA Shapes 6 mm pellets 6 mm pellets 6 mmpellets 9 mm daisy 6 mm pellets 10 mm rings 10 mm rings 10 mm rings 10mm rings 12 mm daisy 12 mm daisy 12 mm daisy 20 mm rings 20 mm rings 14mm rings V₂O₅ 6-8 wt % 7-9 wt % 6-8 wt % — 6-8 wt % content Alkali metal11-15 wt % 11-15 wt % 20-25 wt % — 11-15 wt % oxide (potassium)(potassium) (cesium) (cesium) (potassium) Operating 400-630° C. 400-550°C. 370-450° C. — 400-550° C. temperature

One catalyst may be used alone (e.g. vanadium pentoxide or VK38) or morethan one catalyst may be used in combination according to the propertiesof the individual catalysts.

As mentioned above, the carbon dioxide feed gas comprising SO₂ as acontaminant is preferably flue gas produced by oxyfuel combustion of afuel selected from the group consisting of hydrocarbon fuels andcarbonaceous fuels. However, the Inventors have realized that the methodmay be used in conjunction with existing FGD processes, for example as aretro-fit to the outlet of such processes. In this connection, thecarbon dioxide feed gas may be derived from flue gas produced bycombustion of a fuel selected from the group consisting of hydrocarbonfuels and carbonaceous fuels, in which the flue gas is pre-treated in adesulfurization process to remove a portion of the SO₂ from the flue gasto produce the carbon dioxide feed gas for the method. In suchembodiments, sulfuric acid, typically in the form of an aqueous (mixed)acid solution, may be recycled to the desulfurization process aftersuitable adjustment of the pressure and temperature as required.

In some embodiments, the carbon dioxide feed gas comprising SO₂ as acontaminant may already be at the elevated pressure, e.g. flue gas froma pressurized oxyfuel combustion system. However, in most embodiments,the carbon dioxide gas is compressed to produce the carbon dioxide feedgas at said elevated pressure. The gas may be compressed in a singlestage or in more than one stages, with or without interstage coolingusing heat exchangers. If the gas is compressed in multiple stages, thenintercooling is typically minimal, or even eliminated entirely, sincethe method requires SO₂ oxidation at elevated temperature. Ifintercooling is used, then means (such as “knockout” pots) may be usedto capture any condensate formed during the compression stages.

In preferred embodiments, heat of compression (generated when the carbondioxide gas is compressed to the elevated pressure) alone is sufficientto produce the feed gas at the elevated temperature. However, inembodiments where heat of compression alone is not sufficient, themethod comprises heating the feed gas at the elevated pressure byindirect heat exchange with a heat transfer fluid to produce said carbondioxide feed gas at the elevated temperature.

In a particularly preferred embodiment, there is provided a method forremoving SO₂ and NO_(x) from carbon dioxide feed gas comprising SO₂ andNO_(x) as contaminants. The method comprises contacting the carbondioxide feed gas at an elevated temperature and an elevated pressurewith a catalyst for oxidizing SO₂, in the presence of O₂ to convert SO₂to SO₃, and produce SO₃-enriched carbon dioxide gas comprising NO_(x).The SO₃-enriched carbon dioxide gas comprising NO_(x) is cooled to areduced temperature that is less than the elevated temperature and nomore that the acid dew point at the elevated pressure. The gas ismaintained at the reduced temperature and elevated pressure(s) in thepresence of O₂ and water for a period of time sufficient to convertNO_(x) to nitric acid, thereby producing SO₂-depleted, NO_(x)-leancarbon dioxide gas and an aqueous mixed acid solution comprisingsulfuric and nitric acids. The aqueous mixed acid solution is separatedfrom said SO₂-depleted, NO_(x)-lean carbon dioxide gas, or from aSO₂-depleted, NO_(x)-lean carbon dioxide gas derived therefrom.

One of the advantages of preferred embodiments of the present inventionis that the method works with concentrations of NO_(x) as low as about100 ppm. The concentration of NO_(x) in the carbon dioxide feed gas maybe from about 100 ppm to about 10,000 ppm. In embodiments where thecarbon dioxide feed gas does not comprise NO_(x) as a contaminant, themethod may further comprise adding to the carbon dioxide gas at least aminimum amount of NO_(x) required to provide significant assistance inconverting SO₂ to sulfuric acid. In those embodiments, the amount ofNO_(x) added may be from about 100 ppm to about 10,000 ppm.

An additional advantage of the embodiments of the present inventionremoving SO₂ and NO_(x) from carbon dioxide gas, is that overall reactorvolume, relative to the oxyfuel sour compression process described in US2007/0122328 A1, is reduced.

A further advantage of the embodiments of the present invention removingSO₂ and NO_(x) from carbon dioxide gas, is that the amount (or volume),relative to the oxyfuel sour compression process described in US2007/0122328 A1, of aqueous mixed acid solution produced by the methodcould be reduced depending on the extent to which water needs to beadded to enable acid formation.

The aqueous (mixed) acid solution produced by the method may be used inother commercial processes, e.g. in the production of gypsum (calciumsulfate dihydrate or CaSO₄.2H₂O) from limestone (calcium carbonate orCaCO₃). The more concentrated the aqueous (mixed) acid solution, themore likely the solution would find beneficial commercial uses. In thisconnection, a further advantage of the present invention over the priorart is that the concentration of aqueous (mixed) acid solution may bevaried as desired by controlling the amount of water added to theSO₃-enriched carbon dioxide gas.

Production of aqueous acid solution by a condensation process usuallyresults in the formation of acid mist which can be removed by passingthe SO₂-depleted (NO_(x)-lean) carbon dioxide gas at elevated pressurethrough at least one fiber bed mist eliminator.

At least a portion of the SO₂-depleted (N_(x)-lean) carbon dioxide gasproduced by the present invention may be further processed. In preferredembodiments in which the gas comprises water vapor and “non-condensable”gases such as N₂, O₂ and Ar, the SO_(x)-depleted (N_(x)-lean) carbondioxide gas is usually dried, purified to remove “non-condensable”components, and compressed to a pipeline pressure from about 80 bar toabout 250 bar The gas may then be stored in geological formations or indeep sea locations, or may be used in EOR processes.

The SO₂-depleted (NO_(x)-lean) carbon dioxide gas may be dried in adesiccant drier and then cooled to a temperature close to its triplepoint where “non-condensable” components such as N₂, O₂ and Ar areremoved as gases in a vent stream. This process allows the CO₂ loss withthe vent stream to be minimized by fixing the feed gas pressure at anappropriate level, e.g. from about 20 bar to about 40 bar (2 MPa to 4MPa).

Suitable “non-condensable” components removal processes for use with thepresent invention are described in “Oxyfuel conversion of heaters andboilers for CO ₂ capture” (Wilkinson et al., Second National Conferenceon Carbon Sequestration; May 5-8, 2003; Washington D.C.); US2008/0173584 A1; US 2008/0173585 A1; and US 2008/0176174 A1, thedisclosure of each of which is incorporated herein by reference. If thepresent method is used to remove SO₂ and NO_(x) from flue gas producedin an oxyfuel combustion process and is integrated with one of these“non-condensable” components removal methods, then the integratedprocess typically leads to CO₂ purities of 95% to 99.99%, and to CO₂recoveries of 90% to 99%.

The apparatus comprises a compressor arrangement for elevating thepressure of the carbon dioxide feed gas comprising SO₂ as a contaminant;and a catalytic reactor system comprising a catalyst for oxidizing SO₂,the reactor system being suitable for contacting the carbon dioxide feedgas at an elevated temperature and an elevated pressure with thecatalyst in the presence of O₂ to convert SO₂ to SO₃ and produceSO₃-enriched carbon dioxide gas; together with a conduit arrangement forfeeding the carbon dioxide feed gas at the elevated pressure from thecompressor arrangement to the catalytic reactor system. The apparatusalso comprises a separator system for contacting the SO₃ in theSO₃-enriched carbon dioxide gas with water to produce sulfuric acid andSO₂-depleted carbon dioxide gas, and for separating the sulfuric acidfrom the SO₂-depleted carbon dioxide gas, or from a SO₂-depleted carbondioxide gas derived therefrom; and a conduit arrangement for feedingSO₃-enriched carbon dioxide gas from the catalytic reactor system to theseparator system.

The compressor arrangement may involve a single or multiple stages. Ifthe compressor arrangement involves multiple stages, it may furthercomprise a heat exchanger (or intercooler) for cooling the gas at eachinterstage by indirect heat exchange against a coolant. If multistagecompression intercoolers are present, then an arrangement (e.g.“knockout” pots) should be provided to capture and remove any condensatethat may form during the cooling. However, since the method is tooperate at an elevated temperature, preferred embodiments of theapparatus may be without such compression intercoolers.

Pressure drop through the catalytic reactor system is not a constraintin the present invention (as it is typically for catalytic reactorsoperating at or near atmospheric pressure as in the SNOX™ process) sincethe carbon dioxide gas is already at an elevated pressure suitable forsubsequent CO₂ capture and, thus, a broader range of reactor designs(e.g. packed bed) can be contemplated. In preferred embodiments, thecatalytic reactor system comprises a packed bed of catalyst, or catalystdeposited on the surface of a structured support, e.g. a ceramicmonolith.

In preferred embodiments, the separator system may be operated at thesame elevated pressure as the catalytic reactor system, subject to anyinherent pressure drop between the two systems. However, in embodimentsin which the separator system is intended to operate at a differentelevated pressure to the catalytic reactor system, the apparatus furthercomprises a pressure adjustment arrangement, e.g. an expander or acompressor, in the conduit arrangement from the catalytic reactor systemto the separator system, to adjust the pressure of the SO₃-enrichedcarbon dioxide gas as required.

The separator system may comprise at least one countercurrent gas/liquidcontact device for washing the SO₃-enriched carbon dioxide gas withwater. The contact device may comprise a first inlet for feedingSO₃-enriched carbon dioxide gas into the contact device; a first outletfor removing SO₂-depleted carbon dioxide gas from said contact device; asecond inlet for feeding water from an external source into said contactdevice; and a second outlet for removing sulfuric acid from the contactdevice in the form of an aqueous acid solution.

Where the contact device is a mass/heat transfer separation column (e.g.a “scrub” tower or “spray” tower), aqueous (mixed) acid solution isusually recycled to the top of the column or tower. The recycled portionof the aqueous solution is usually pumped to a higher pressure toproduce a pumped solution which is then cooled before recycling.

Additionally or alternatively, the separator system may comprise atleast one acid condenser for cooling and condensing sulfuric acid byindirect heat exchange with a coolant. The condenser may have ashell-and-tube type structure with a tube-side for the SO₃-enrichedcarbon dioxide gas and a shell-side for the coolant. The acid condensermay comprise a first inlet for feeding SO₃-enriched carbon dioxide gasto the tube side of the condenser; a first outlet for removingSO₂-depleted carbon dioxide gas from the tube-side of the condenser; asecond inlet for feeding coolant to the shell-side of the condenser; asecond outlet for removing coolant from the shell-side of the condenser;a third inlet for feeding water from an external source to the tube-sideof the condenser; and a third outlet for removing sulfuric acid from thetube-side of the condenser in the form of an aqueous acid solution.

A suitable (mixed) acid condenser may be a WSA condenser. However, sincethe condensation would take place at elevated pressure, it may benecessary to use a modified WSA-type condenser, designed to operate atthe higher pressures.

If the temperature of the gas leaving the compression arrangement isless than the required elevated temperature for any reason (e.g. theapparatus comprises multistage compression intercoolers, or heat ofcompression alone is not sufficient to reach the required elevatedtemperature), then the conduit arrangement for feeding carbon dioxidefeed gas at an elevated pressure from the compressor arrangement to saidcatalytic reactor system, may comprise a heat exchanger arrangement forheating the carbon dioxide gas at the elevated pressure to the elevatedtemperature by indirect heat exchange with a heat transfer fluid, e.g.steam.

In embodiments of the apparatus for removing NO_(x) in addition to SO₂from carbon dioxide feed gas comprising NO_(x) as a further contaminant,nitric acid is produced and separated in the separator system withsulfuric acid, usually in the form of an aqueous mixed acid solution, toproduce SO₂-depleted, NO_(x)-lean carbon dioxide gas. In suchembodiments, the separator system maintains the SO₃-enriched carbondioxide comprising NO_(x) at elevated pressure(s) in the presence of O₂and water for a period of time sufficient to convert NO_(x) to nitricacid via the sour compression reactions.

The separator system may comprise a reactor system for the sourcompression reactions and a convention gas/liquid separationarrangement. Such a reactor system simply provides a sufficient volumefor a given flow rate within which the reactions may take place atelevated pressure(s). The reactor system usually comprises at least onepressurizable reactor vessel such as a pipe or duct; a tank; anabsorption column; a wet scrubbing tower; fluidized or moving bed;packed tower or column; and a Venturi scrubber. Where the reactor systemcomprises a countercurrent gas/liquid contact column, acid condensatemay be removed from the bottom of the column, pumped, cooled and fed asreflux to the top of the column.

The reactor system may comprise a single pressurizable reactor vesselfor operation at a single elevated pressure within the range of suitablepressures. In other embodiments, the reactor system may comprise atleast two (same or different) pressurizable reactor vessels foroperation at either the same or different elevated pressures.

Where there are at least two reactor vessels for operation at differentelevated pressures, a gas compression arrangement may be provided tocompress the gaseous effluent from the elevated operating pressure of afirst vessel to the elevated operating pressure of a second vessel. Thegas compression arrangement may be at least one stage of a multiplestage gas compressor.

In a particularly preferred embodiment, the reactor system of theseparator system comprises a first gas/liquid contact column foroperation at a first elevated pressure, and a second gas/liquid contactcolumn for operation at a second elevated pressure that is higher thanthe first elevated pressure. The first and second elevated pressures aretypically both within the preferred ranges given above. The firstelevated pressure may be from about 10 bar to about 20 bar (1 MPa to 2MPa) and the second elevated pressure may be from about 25 bar to about35 bar (2.5 MPa to 3.5 MPa). Where both SO₂ and NO_(x) are present ascontaminants in the carbon dioxide gas to be processed in suchembodiments, both columns usually produce mixed acid condensate with thefirst column producing predominantly sulfuric acid condensate and thesecond column producing predominantly nitric acid condensate.

The apparatus may comprise an expander arrangement for reducing thepressure of an aqueous (mixed) acid solution comprising sulfuric acid(and nitric acid) to produce the aqueous (mixed) acid solution atreduced pressure; a conduit arrangement for feeding the aqueous (mixed)acid solution from the separator system to the expander arrangement; anda conduit arrangement for feeding the aqueous (mixed) acid solution atreduced pressure from the expander to a flue gas desulfurization system.

In preferred embodiments, the apparatus is integrated with an oxyfuelcombustion system. In these embodiments, the apparatus may comprise anoxyfuel combustion system for combusting a fuel selected fromhydrocarbon fuels and carbonaceous fuels, in the presence of essentiallypure oxygen to produce flue gas, a portion of which optionally beingrecycled to the oxyfuel combustion system; a wash system for washingflue gas with water to produce washed flue gas; a conduit arrangementfor feeding flue gas from the oxyfuel combustion system to the washsystem; and a conduit arrangement for feeding washed flue gas form thewash system to the compressor arrangement.

Since the proposed invention would substantially reduce theconcentration of SO₂ in, or even eliminate SO₂ from, the flue gas froman oxyfuel combustion process, conventional equipment for FGD processesto remove SO₂ can be substantially reduced in size or even eliminatedaccordingly. In addition, since embodiments of the proposed inventionwould substantially reduce the concentration of NO_(x) in the flue gasfrom such a process, conventional equipment for an SCR (e.g. a deNO_(x)system) to remove NO₂ can also be substantially reduced or eveneliminated.

The apparatus may further comprise a drier arrangement to dry theSO_(x)-depleted (NO_(x)-lean) carbon dioxide gas and produce driedSO₂-depleted (NO_(x)-lean) carbon dioxide gas; and a “non-condensable”components separation system to remove “non-condensable” components suchas O₂, N₂ and Ar from the dried gas. Suitable combinations of a drierarrangement and a “non-condensable” components separation system aredisclosed in US 2008/0173584 A1; US 2008/0173585 A1; and US 2008/0176174A1.

Aspects of the invention include:

#1. A method for removing SO₂ from a carbon dioxide feed gas comprisingSO₂ as a contaminant, said method comprising:

-   -   contacting said carbon dioxide feed gas at an elevated        temperature and an elevated pressure with a catalyst for        oxidizing SO₂, in the presence of O₂ to convert SO₂ to SO₃ and        produce an SO₃-enriched carbon dioxide gas; and    -   contacting SO₃ in said SO₃-enriched carbon dioxide gas with        water to produce sulfuric acid and a SO₂-depleted carbon dioxide        gas; and    -   separating said sulfuric acid from said SO₂-depleted carbon        dioxide gas, or from a SO₂-depleted carbon dioxide gas derived        therefrom.        #2. A method according to #1, wherein the elevated pressure is        at least about 2 bar (0.2 MPa).        #3. A method according to #1 or #2, wherein the elevated        pressure is no more than about 50 bar (5 MPa).        #4. A method according to any of #1 to #3, wherein the elevated        temperature is at least 300° C.        #5. A method according to any of #1 to #4, wherein the elevated        temperature is no more than about 700° C.        #6. A method according to any of #1 to #5, wherein said carbon        dioxide feed gas comprises NO_(x) as a further contaminant, said        method additionally producing nitric acid for separation with        said sulfuric acid from said SO₂-depleted carbon dioxide gas        which is also NO_(x)-lean, or from said gas derived therefrom.        #7. A method according to any of #1 to #6, said method        comprising cooling said SO₃-enriched carbon dioxide gas to a        reduced temperature that is less than the elevated temperature        and no more that the acid dew point at said elevated pressure,        thereby condensing sulfuric acid, wherein said sulfuric acid is        separated from said SO₂-depleted carbon dioxide gas, or a gas        derived therefrom, in the form of a liquid.        #8. A method according to #7, wherein said reduced temperature        is no more than 300° C.        #9. A method according to #7 or #8, wherein said SO₃-enriched        carbon dioxide gas is cooled by indirect heat exchange against        at least one coolant.        #10. A method according to any of #7 to #9, wherein said        SO₃-enriched carbon dioxide gas is cooled by direct heat        exchange with water from an external source.        #11. A method according to any of #7 to #10, wherein said carbon        dioxide feed gas comprises NO_(x) as a further contaminant, said        method additionally producing nitric acid for separation with        said sulfuric acid from said SO₂-depleted carbon dioxide gas        which is also NO_(x)-lean, or from said gas derived therefrom.        #12. A method according to #11, said method comprising        maintaining said SO₃-enriched carbon dioxide gas comprising        NO_(x) at said elevated pressure in the presence of O₂ and water        for a period of time sufficient to convert NO_(x) to nitric        acid.        #13. A method according to #12, wherein said period of time is        no more than 600 seconds.        #14. A method according to any of #11 to #13, wherein said        catalyst also oxidizes nitric oxide (NO) to NO₂.        #15. A method according to any of #1 to #14, wherein said carbon        dioxide feed gas is contacted with said catalyst in a catalytic        reactor at an volumetric hourly space velocity from about 5,000        to about 500,000 Nm³ _(feed)/h/m³ _(catalyst bed.)        #16. A method according to any of #1 to #15, comprising        compressing carbon dioxide gas comprising SO₂ as a contaminant        to produce said carbon dioxide feed gas at said elevated        pressure, wherein heat of compression alone is sufficient to        produce said carbon dioxide feed gas at said elevated        temperature.        #17. A method according to any of #1 to #16, comprising:    -   compressing carbon dioxide gas comprising SO₂ as a contaminant        to produce said carbon dioxide feed gas at said elevated        pressure; and    -   heating said carbon dioxide feed gas at said elevated pressure        by indirect heat exchange with a heat transfer fluid to produce        said carbon dioxide feed gas at said elevated temperature.        #18. A method according to any of #1 to #17, wherein said carbon        dioxide feed gas is, or is derived from, flue gas produced by        oxyfuel combustion of a fuel selected from the group consisting        of hydrocarbon fuels and carbonaceous fuels.        #19. A method according to #18, wherein said flue gas is        pre-treated in a desulfurization process to remove a portion of        the SO₂ from the flue gas.        #20. A method according to #19, wherein said sulfuric acid is        recycled to the desulfurization process after suitable        adjustment of the pressure and temperature as required.        #21. A method for removing SO₂ and NO_(x) from carbon dioxide        feed gas comprising SO₂ and NO_(x) as contaminants, said method        comprising:    -   contacting said carbon dioxide feed gas at an elevated        temperature and an elevated pressure with a catalyst for        oxidizing SO₂, in the presence of O₂ to convert SO₂ to SO₃ and        produce SO₃-enriched carbon dioxide gas comprising NO_(x);    -   cooling said SO₃-enriched carbon dioxide gas comprising NO_(x)        to a reduced temperature that is less than the elevated        temperature and no more that the acid dew point at said elevated        pressure, wherein said gas is maintained at said reduced        temperature and elevated pressure(s) in the presence of O₂ and        water for a period of time sufficient to convert NO_(x) to        nitric acid, thereby producing SO₂-depleted, NO_(x)-lean carbon        dioxide gas and an aqueous mixed acid solution comprising        sulfuric and nitric acids; and    -   separating said aqueous mixed acid solution from said        SO₂-depleted, NO_(x)-lean carbon dioxide gas, or from a        SO₂-depleted, NO_(x)-lean carbon dioxide gas derived therefrom.        #22. Apparatus for removing SO₂ from carbon dioxide feed gas        comprising SO₂ as a contaminant, said apparatus comprising:    -   a compressor arrangement for elevating the pressure of carbon        dioxide feed gas comprising SO₂ as a contaminant;    -   a catalytic reactor system comprising a catalyst for oxidizing        SO₂, said reactor system being suitable for contacting carbon        dioxide feed gas at an elevated temperature and an elevated        pressure with said catalyst in the presence of O₂ to convert SO₂        to SO₃ and produce SO₃-enriched carbon dioxide gas;    -   a conduit arrangement for feeding carbon dioxide feed gas at an        elevated pressure from said compressor arrangement to said        catalytic reactor system;    -   a separator system for contacting SO₃ in said SO₃-enriched        carbon dioxide gas with water to produce sulfuric acid and        SO₂-depleted carbon dioxide gas, and for separating said        sulfuric acid from said SO₂-depleted carbon dioxide gas, or from        a SO₂-depleted carbon dioxide gas derived therefrom; and    -   a conduit arrangement for feeding SO₃-enriched carbon dioxide        gas from said catalytic reactor system to said separator system.        #23. An apparatus according to #22 for removing NO_(x) in        addition to SO₂ from said carbon dioxide feed gas comprising        NO_(x) as a further contaminant, wherein nitric acid is produced        and separated in the separator system with sulfuric acid to        produce SO₂-depleted, NO_(x)-lean carbon dioxide gas.        #24. An apparatus according to #23, wherein said separator        system maintains SO₃-enriched carbon dioxide comprising NO_(x)        at said elevated pressure in the presence of O₂ and water for a        period of time sufficient to convert NO_(x) to nitric acid.        #25. An apparatus according to any of #22 to #24 comprising    -   an expander arrangement for reducing the pressure of an aqueous        acid solution comprising said sulfuric acid to produce said        aqueous acid solution at reduced pressure;    -   a conduit arrangement for feeding said aqueous acid solution        from said separator system to said expander arrangement; and    -   a conduit arrangement for feeding said aqueous acid solution at        reduced pressure from said expander to a flue gas        desulfurization system.

Referring to FIG. 1, a stream 2 of flue gas (comprising about 83% CO₂and SO₂, NO_(x) and O₂ as contaminants) from an oxyfuel combustor system(not shown) at a temperature of about 170° C. is compressed in acompressor 4 to produce a stream 6 of compressed flue gas at an elevatedpressure of about 10 bar (1 MPa). The compressor 4 has at least twocompression stages and raises the temperature of the flue gas duringcompression to an elevated temperature of about 450° C.

Stream 6 is fed to a catalytic reactor system 8 comprising a firstpressurized reactor vessel 10 and a packed bed 12 of vanadiumpentoxide-based catalyst. The specific catalyst used in this example isVK38 (12 mm daisy) manufactured by Haldor Topsøe A/S. In reactor system8, SO₂ is converted to SO₃ by catalytic oxidation of SO₂ with O₂.

A stream 14 of SO₃-enriched carbon dioxide gas comprising NO_(x) isremoved from the reactor system 8 and fed to a cooler 16 where it iscooled to about 20° C. by indirect heat exchange against a coolant, e.g.cooling water, to produce a cooled stream 18 of SO₃-enriched carbondioxide gas comprising NO_(x). Stream 18 is then fed to a separationsystem which includes a second pressurized reactor vessel 20 and apressurized gas/liquid separator 26.

The SO₃-enriched carbon dioxide gas comprising NO_(x) is maintained inreactor vessel 20 at the elevated pressure in the presence of oxygen andwater for a period of time sufficient to not only convert NO_(x) tonitric acid but also SO₃ and residual SO₂ to sulfuric acid, by the sourcompression reactions (ii) to (vi) mentioned above. A stream 22 of waterfrom an external source may be added to the reactor vessel 20 duringthis step to facilitate production of an aqueous mixed acid solution ofnitric and sulfuric acids.

A stream 24 comprising SO₂-depleted, NO_(x)-lean carbon dioxide gas andthe aqueous mixed acid solution is fed to the gas/liquid separator 26and separated to produce a stream 28 of SO₂-depleted, NO_(x)-lean carbondioxide gas and a stream 30 of the aqueous mixed acid solution. Stream28 may be fed to a drier arrangement and “non-condensable” gasesseparation train of a CO₂ recovery and purification system (not shown).Stream 30 may be fed to a process (not shown) for producing gypsum fromlimestone.

Reactor vessel 20 and gas/liquid separator 26 are depicted in FIG. 1 asseparate components of the separator system. However, it should be notedthat depicting these components in this way should not be interpreted asmeaning that these components must be separate. Some embodiments of theinvention may indeed have a gas/liquid separator 26 that is separatefrom the reactor vessel 20. However, in other embodiments, theseparating of gas and liquid takes place in the reactor vessel 20itself, in which case streams 28 and 30 are taken directly from thereactor vessel 20. Such an embodiment is depicted in FIG. 2.

If the heat of compression alone is not sufficient to raise thetemperature of stream 2 to the required elevated temperature, a heatexchanger 32 may be used to heat stream 6 by indirect heat exchangeusing a heat transfer fluid, e.g. steam. Such an embodiment is depictedin FIG. 3. In one example of this embodiment, the temperature of stream2 is ambient (˜25° C.). Heat of compression from compressor 4 raises thetemperature of the gas such that stream 6 is at a temperature of about247° C. Heat exchanger 32 raises the temperature of the gas at theelevated pressure to the elevated temperature, e.g. 450° C.

The features that are common between FIG. 1 and FIGS. 2 and 3 have beengiven the same numerical references.

The process depicted in any of FIGS. 1 to 3 may be retro-fitted to anexisting SNOX™ process. In such an embodiment, stream 2 would be takenfrom the WSA-condenser (not shown) and the pressure of stream 30 wouldbe reduced in an expander (not shown) and then returned to theWSA-condenser.

Example

Computer simulations using the ASPEN™ Plus software (version 2006.5; ©Aspen Technology, Inc.) have been carried out to compare the processdepicted in FIG. 1 (Cases J through L) with corresponding processes onlyinvolving either the catalytic oxidation of SO₂ in reactor 8 (Cases Athough F) or the non-heterogeneous catalytic conversion of SO₂ andNO_(x) to sulfuric acid and nitric acid respectively in reactor 20(Cases G through I).

In the simulations, the carbon dioxide feed gas had the followingcomposition: 82.37% CO₂, 8.5% N₂, 4.5% O₂, 2.5% H₂O, 2% Ar, 1000 ppmSO₂, and 300 ppm NO. The elevated pressure was 10 bar (1 MPa), theelevated temperature was 450° C. and the reduced temperature was 20° C.For the purpose of the simulations, it was assumed that:

-   -   the catalytic reaction is SO₂+½O₂→SO₃;    -   there is no conversion of NO to NO₂ in the catalytic reactor;        and    -   the catalytic reaction is 1^(st) order with respect to SO₂ and        zero order with respect to O₂.

The 1^(st) order rate constant for the catalytic SO₂ oxidation reactionwas estimated from Examples 4 to 7 and Table 2 in U.S. Pat. No.4,781,902 to be about 3.71 sec^(−1 @450)° C.

The results of the various simulations (Cases A through L) are providedin Table 3.

It should be noted that a given residence time at 1 bar representseither a 10× larger reactor or a 10× lower mass flow rate than the sameresidence time at 10 bar For example, Cases D and F have the sameresidence time in reactor 8 but Case F has a 10× higher mass flow ratedue to the higher pressure.

The results indicate that the combined system (reactors 8 and 20 incombination) provides a higher SO₂ conversion rate at the same residencetime (compare Case K to Case E or Case H). In addition, it may beobserved that the combined system provides the same SO₂ conversion atless than ⅓ of the catalyst amount (compare Case K to Case C). Further,the combined system provides the same conversion with a reactor 20 ofhalf the size (compare Case J to Case H).

TABLE 3 Resi- Volumetric Resi- Pres- dence space velocity dence suretime in in reactor 8 time in SO₂ NO_(x) in bar reactor (Nm³ _(feed)/reactor conv. conv Case (MPa) 8 (s) h/m³ _(cat.)) 20 (s) (%) (%) Reactor8 only: A   1 (0.1) 0.17 9378 N/A 46.4 0.0 B   1 (0.1) 0.33 4831 N/A71.9 0.0 C   1 (0.1) 0.67 2380 N/A 93.2 0.0 D   1 (0.1) 1.34 1190 N/A99.1 0.0 E 10 (1) 0.16 99646 N/A 45.9 0.0 F 10 (1) 1.31 12170 N/A 99.40.0 Reactor 20 only: G 10 (1) N/A N/A  53.9 79.0 29.9 H 10 (1) N/A N/A107.1 89.4 48.4 I 10 (1) N/A N/A 532.0 98.2 81.8 Reactors 8 and 20 J 10(1) 0.10 159433  54.1 88.0 41.0 K 10 (1) 0.20 79717 108.1 96.5 64.9 L 10(1) 1.00 15943 546.5 100.0 91.3

It will be appreciated that the invention is not restricted to thedetails described above with reference to the preferred embodiments butthat numerous modifications and variations can be made without departingform the spirit or scope of the invention as defined in the followingclaims.

1. A method for removing sulfur dioxide (SO₂) from a carbon dioxide feedgas comprising SO₂ as a contaminant, said method comprising: contactingsaid carbon dioxide feed gas at an elevated temperature and an elevatedpressure with a heterogeneous catalyst for oxidizing SO₂, in thepresence of oxygen (O₂) to convert SO₂ to sulfur trioxide (SO₃) andproduce an SO₃-enriched carbon dioxide gas; contacting SO₃ in saidSO₃-enriched carbon dioxide gas with water to produce sulfuric acid anda SO₂-depleted carbon dioxide gas; and separating said sulfuric acidfrom said SO₂-depleted carbon dioxide gas, or from a SO₂-depleted carbondioxide gas derived therefrom.
 2. The method of claim 1, wherein theelevated pressure is at least about 2 bar (0.2 MPa).
 3. The method ofclaim 1, wherein the elevated pressure is no more than about 50 bar (5MPa).
 4. The method of claim 1, wherein the elevated temperature is atleast 300° C.
 5. The method of claim 1, wherein the elevated temperatureis no more than about 700° C.
 6. The method of claim 1, wherein saidcarbon dioxide feed gas comprises NO_(x) as a further contaminant, saidmethod additionally producing nitric acid for separation with saidsulfuric acid from said SO₂-depleted carbon dioxide gas which is alsoNO_(x)-lean, or from said gas derived therefrom.
 7. The method of claim1, said method comprising cooling said SO₃-enriched carbon dioxide gasto a reduced temperature that is less than the elevated temperature andno more than the acid dew point at said elevated pressure, therebycondensing sulfuric acid, wherein said sulfuric acid is separated fromsaid SO₂-depleted carbon dioxide gas, or said gas derived therefrom, inthe form of a liquid.
 8. The method of claim 7, wherein said reducedtemperature is no more than 300° C.
 9. The method of claim 7, whereinsaid SO₃-enriched carbon dioxide gas is cooled by indirect heat exchangeagainst at least one coolant.
 10. The method of claim 7, wherein saidSO₃-enriched carbon dioxide gas is cooled by direct heat exchange withwater from an external source.
 11. The method of claim 7, wherein saidcarbon dioxide feed gas comprises NO as a further contaminant, saidmethod additionally producing nitric acid for separation with saidsulfuric acid from said SO₂-depleted carbon dioxide gas which is alsoNO_(x)-lean, or from said gas derived therefrom.
 12. The method of claim11, said method comprising maintaining said SO₃-enriched carbon dioxidegas comprising NO_(x) at said elevated pressure in the presence of O₂and water for a period of time sufficient to convert NO_(x) to nitricacid.
 13. The method of claim 12, wherein said period of time is no morethan 600 seconds.
 14. The method of claim 11, wherein said catalyst alsooxidizes nitric oxide (NO) to NO₂.
 15. The method of claim 1, whereinsaid carbon dioxide feed gas is contacted with said catalyst in acatalytic reactor at an volumetric hourly space velocity from about5,000 to about 500,000 Nm³ _(feed)/h/m³ _(catalyst bed.)
 16. The methodof claim 1, comprising compressing carbon dioxide gas comprising SO₂ asa contaminant to produce said carbon dioxide feed gas at said elevatedpressure, wherein heat of compression alone is sufficient to producesaid carbon dioxide feed gas at said elevated temperature.
 17. Themethod of claim 1, comprising: compressing carbon dioxide gas comprisingSO₂ as a contaminant to produce said carbon dioxide feed gas at saidelevated pressure; and heating said carbon dioxide feed gas at saidelevated pressure by indirect heat exchange with a heat transfer fluidto produce said carbon dioxide feed gas at said elevated temperature.18. The method of claim 1, wherein said carbon dioxide feed gas is, oris derived from, flue gas produced by oxyfuel combustion of a fuelselected from the group consisting of hydrocarbon fuels and carbonaceousfuels.
 19. The method of claim 18, wherein said flue gas is pre-treatedin a desulfurization process to remove a portion of the SO₂ from theflue gas.
 20. The method of claim 19, wherein said sulfuric acid isrecycled to the desulfurization process after suitable adjustment of thepressure and temperature as required.
 21. A method for removing SO₂ andNO_(x) from carbon dioxide feed gas comprising SO₂ and NO_(x) ascontaminants, said method comprising: contacting said carbon dioxidefeed gas at an elevated temperature and an elevated pressure with acatalyst for oxidizing SO₂, in the presence of O₂ to convert SO₂ to SO₃and produce SO₃-enriched carbon dioxide gas comprising NO_(x); coolingsaid SO₃-enriched carbon dioxide gas comprising NO_(x) to a reducedtemperature that is less than the elevated temperature and no more thatthe acid dew point at said elevated pressure, wherein said gas ismaintained at said reduced temperature and elevated pressure(s) in thepresence of O₂ and water for a period of time sufficient to convertNO_(x) to nitric acid, thereby producing SO₂-depleted, NO_(x)-leancarbon dioxide gas and an aqueous mixed acid solution comprisingsulfuric and nitric acids; and separating said aqueous mixed acidsolution from said SO₂-depleted, NO_(x)-lean carbon dioxide gas, or froma SO₂-depleted, NO_(x)-lean carbon dioxide gas derived therefrom. 22.Apparatus for removing SO₂ from carbon dioxide feed gas comprising SO₂as a contaminant, said apparatus comprising: a compressor arrangementfor elevating the pressure of carbon dioxide feed gas comprising SO₂ asa contaminant; a catalytic reactor system comprising a catalyst foroxidizing SO₂, said reactor system being suitable for contacting carbondioxide feed gas at an elevated temperature and an elevated pressurewith said catalyst in the presence of O₂ to convert SO₂ to SO₃ andproduce SO₃-enriched carbon dioxide gas; a conduit arrangement forfeeding carbon dioxide feed gas at an elevated pressure from saidcompressor arrangement to said catalytic reactor system; a separatorsystem for contacting SO₃ in said SO₃-enriched carbon dioxide gas withwater to produce sulfuric acid and SO₂-depleted carbon dioxide gas, andfor separating said sulfuric acid from said SO₂-depleted carbon dioxidegas, or from a SO₂-depleted, NO_(x)-lean carbon dioxide gas derivedtherefrom; and a conduit arrangement for feeding SO₃-enriched carbondioxide gas from said catalytic reactor system to said separator system.23. The apparatus of claim 22 for removing NO_(x) in addition to SO₂from said carbon dioxide feed gas comprising NO_(x) as a furthercontaminant, wherein nitric acid is produced and separated in theseparator system with sulfuric acid to produce SO₂-depleted, NO_(x)-leancarbon dioxide gas.
 24. The apparatus of claim 23, wherein saidseparator system maintains SO₃-enriched carbon dioxide comprising NO_(x)at elevated pressure(s) in the presence of O₂ and water for a period oftime sufficient to convert NO_(x) to nitric acid.
 25. The apparatus ofclaim 22 comprising an expander arrangement for reducing the pressure ofan aqueous acid solution comprising said sulfuric acid to produce saidaqueous acid solution at reduced pressure; a conduit arrangement forfeeding said aqueous acid solution from said separator system to saidexpander arrangement; and a conduit arrangement for feeding said aqueousacid solution at reduced pressure from said expander to a flue gasdesulfurization system.